Diodes ZNBG3211 - Manual (Page 8)

FUNCTIONAL DIAGRAM
FUNCTIONAL DESCRIPTION
The ZNBG devices provide all the bias requirements for external FETs, including the generation
of the negative supply required for gate biasing, from the single supply voltage.The diagram
above shows a single stage from the ZNBG series. The ZNBG3210/11 contains 3 such stages. The
negative rail generator is common to all devices.
The drain voltage of the external FET Q
N
is set by the ZNBG device to its normal operating voltage.
This is determined by the on board V
D
Set reference, for the ZNBG3210 this is nominally 2.2 volts
whilst the ZNBG3211 provides nominally 2 volts.
The drain current taken by the FET is monitored by the low value resistor ID Sense. The amplifier
driving the gate of the FET adjusts the gate voltage of Q
N
so that the drain current taken matches
the current called for by an external resistor R
CAL
.
Since the FET is a depletion mode transistor, it is often necessary to drive its gate negative with
respect to ground to obtain the required drain current. To provide this capability powered from
a single positive supply, the device includes a low current negative supply generator. This
generator uses an internal oscillator and two external capacitors, C
NB
and C
SUB
.
67-8
ZNBG3210
ZNBG3211